The storage principle of sorption or adsorption accumulators is based on the property of some highly porous materials, such as for example silica gel, of attracting water vapor and bonding to the surface of the material, releasing heat. This accumulation of water is referred to as adsorption. Adsorption accumulators of this type are often used as drying agents in packaging materials. Conversely, when the material is heated, the bonded water is released again, or desorbed, in the form of water vapor, while at the same time the accumulator is laden with thermal energy. This process can be repeated as often as desired. Adsorption accumulators can store thermal energy in a high density.
Adsorption accumulators are used in stationary heating engineering, where they are employed in particular to improve the energy balance of solar thermal installations and district heating systems, in that they are responsible for balancing the thermal energy in the event of fluctuations over the course of time.
For mobile applications, in particular to assist with what is known as a cold start, it is customary to use heating systems which are based on thermally insulated hot water reservoirs (known as sensible heat storage), on accumulators which make use of the phase change of a material (known as latent heat storage) or on mobile incineration systems or electrical heating systems (stationary heating). Laid-open specification WO 02/054520 A1 relates, for example, to the use of a latent heat store in a mobile fuel cell system.
Adsorption accumulators have also found their way into mobile application areas. For example, laid-open specification DE 43 10 836 A1 has disclosed the provision of adsorption accumulators in motor vehicles driven by internal combustion engines; the thermal energy stored by the adsorption accumulator can be utilized to heat the vehicle interior or also an internal combustion engine which drives the vehicle prior to a start.
Patent application JP 10-144333 uses an adsorption accumulator to heat a fuel cell unit of a motor vehicle. The adsorption accumulator is assigned a condenser/evaporator, in which water is available for discharging the adsorption accumulator. When the fuel cell unit has reached an appropriate operating temperature, the adsorption accumulator, together with the condenser and two downstream heat exchangers, functions as cooling for the fuel cell unit, one of the heat exchangers releasing the heat of the fuel cell unit to the ambient air. The system described, in terms of the water balance, is a closed system in which there is no exchange of water with the environment. Water for discharging the adsorption accumulator is permanently available in the condenser. For this reason, with the system described, a start is only possible at temperatures above freezing point.